Currently, in 3GPP (3rd Generation Partnership Project), the W-CDMA system has been standardized as a 3G cellular mobile communication system, and its service has been started sequentially. Further, HSDPA (High Speed Downlink Packet Access) with the communication speed further increased has also been standardized, and its service is being started.
Meanwhile, in 3GPP, evolution in 3rd Generation Radio Access (Evolved Universal Terrestrial Radio Access: hereinafter, referred to as “EUTRA”) has been studied. As downlink in the EUTRA, an OFDM (Orthogonal Frequency Division Multiplexing) system is proposed. Further, proposed as uplink in the EUTRA is a DFT (Discrete Fourier Transform)-spread OFDM type single carrier communication system.
As shown in FIG. 22, the uplink of EUTRA is formed of an uplink pilot channel UPiCH, random access channel RACH, and uplink scheduling channel USCH (for example, see Non-Patent Document 1).
The uplink random access channel RACH of E-UTRA contains a non-synchronized random access channel and synchronized random access channel. Herein, a band of 1.25 MHz is used as a maximum unit of the non-synchronized random access channel. Then, for example, as shown in FIG. 23, a plurality of channels for access is prepared, and configured to be able to respond to a number of accesses.
Among intended purposes of the non-synchronized random access channel, it is desirable to synchronize a mobile station apparatus (hereinafter, referred as a “mobile station”) and base station apparatus (hereafter, referred to as a “base station”). Further, it is considered that a mobile station transmits several-bit information to request scheduling for allocating radio resource and the like so as to decrease the connection time between the mobile station and base station. Meanwhile, the intended purpose of the synchronized random access is to make a scheduling request (for example, see Non-patent Document 2).
In the non-synchronized random access, only a preamble is transmitted to acquire synchronization. This preamble contains a signature that is a signal pattern indicative of information, and by preparing a few tens of kinds of signatures, it is possible to designate several-bit information. Currently, it is anticipated that 6-bit information is transmitted, and that 64 kinds of signatures are prepared.
In the 6-bit information, it is expected that 5 bits are assigned a random ID, while remaining 1 bit is assigned a reason of random access, downlink path-loss/CQI (Channel Quality Indicator) and the like (for example, see Non-patent Document 3).
FIG. 24 is a sequence chart to explain an example of a conventional procedure of random access. In addition, FIG. 24 shows the procedure of random access (non-synchronized random access) in the case of using a non-synchronized random access channel.
As shown in FIG. 24, in the conventional procedure of random access, a mobile station first selects a signature based on a random ID, the reason of random access, downlink path-loss/CQI information and the like (step (hereinafter, abbreviated as “ST”) 2401). Then, the mobile station transmits a preamble (random access preamble) containing the selected signature on the non-synchronized random access channel (ST2402:Message 1).
Upon receiving the preamble from the mobile station, the base station calculates a synchronization timing deviation between the mobile station and base station from the preamble, and performs scheduling for transmitting an L2/L3 (Layer2/Layer3) message (ST2403). Then, the base station assigns C-RNTI (Cell-Radio Network Temporary Identity) to the mobile station requiring C-RNTI from the random access reason, and transmits a random access response including synchronization timing deviation information (synchronization information), scheduling information, signature ID number and C-RNTI (ST2404:Message 2).
Upon receiving these pieces of information from the base station, the mobile station extracts the response from the base station including the transmitted signature ID number (ST2405). Then, the mobile station transmits an L2/L3 message with radio resources subjected to scheduling in the base station (ST2406:Message 3). Upon receiving the L2/L3 message from the mobile station, the base station transmits a contention resolution to judge whether a collision occurs between mobile stations to the mobile station (ST2407:Message 4)(for example, see Non-patent Document 3).
A problem of such random access is that a collision occurs in the case that a plurality of different mobile stations selects the same signature and random access channel. When a plurality of mobile stations selects the same signature and transmits the signature with a radio resource block having the same time and frequency i.e. on the same random access channel, a collision occurs in the preamble (ST2402) as shown in FIG. 24.
When the base station cannot detect the preamble (ST2402) due to such a collision, the base station cannot send back the response (ST2404) including the synchronization information and the like to the mobile station. In this case, the mobile station cannot receive the response (ST2404) from the base station, and therefore, needs to select a signature and random access channel again after a lapse of predetermined time to perform random access.
Meanwhile, when the base station can detect the preamble (ST2402), the base station calculates L2/L3 message scheduling and synchronization timing deviation, and sends back a response (ST2404) to the mobile station. However, a plurality of mobile stations receives the response (ST2404) from the base station. Therefore, the plurality of mobile stations transmits the L2/L3 message (ST2406) with radio resources subjected to scheduling, and as a result, the collision occurs in the L2/L3 message (ST2406).
When the base station cannot detect the L2/L3 message (ST2406) due to such a collision, the base station cannot send back the response (ST2407) to the mobile stations. In this case, the mobile stations cannot receive the response (ST2407) from the base station, and therefore, need to select a signature and random access channel again after a lapse of predetermined time to perform random access. Thus, when a plurality of mobile stations selects the same signature and random access channel, the collision can occur, while when the collision occurs, the time up to ST2407 as shown in FIG. 24 is required at the maximum until the collision is detected.
Meanwhile, when a mobile station capable of executing such random access is located in a position as shown in FIG. 25, handover is executed. Also when handover is executed, the above-mentioned non-synchronized random access is performed.
Described herein is an example of a procedure of random access at the time of executing handover. FIG. 26 is a sequence chart to explain an example of a procedure of random access at the time of executing handover. In addition, as in FIG. 24, FIG. 26 shows the procedure of random access in the case of using a non-synchronized random access channel.
As shown in FIG. 26, in the procedure of random access at the time of executing handover, as a preparatory stage of handover, a mobile station first measures radio signal conditions of adjacent base stations (ST2601). Then, the mobile station transmits the measurement result (measurement report) to a base station A that is a base station (hereinafter, referred to as a “local-base station” as appropriate) currently holding the mobile station (ST2602).
Upon receiving the measurement result from the mobile station, the base station A selects an optimal base station from the measurement result (ST2603). In addition, herein, a base station B is assumed to be selected as an optimal base station. Then, the base station A transmits a handover request message to the base station B that is a handover destination (ST2604).
Upon receiving the handover request message from the base station A, the base station B assigns C-RNTI to the mobile station performing handover (ST2605). Then, as a response to the handover request, the base station B notifies the base station A of a handover request acknowledge message including the C-RNTI (ST2606).
Upon receiving the handover request acknowledge message from the base station B, the base station A transmits a handover command message including the C-RNTI to the mobile station (ST2607).
Upon receiving the handover command message from the base station A, the mobile station acquires synchronization on downlink of the base station B, and confirms a position of the random access channel from the broadcast channel (ST2608). When the downlink synchronization is acquired, the mobile station selects one signature from among signatures such that the reason of random access is handover (ST2609). Then, the mobile station transmits a preamble (random access preamble) containing the selected signature to the base station B on the random access channel (ST2610:Message 1).
Upon detecting the signature from the preamble received from the mobile station, the base station B calculates a synchronization timing deviation, and performs scheduling of uplink for the mobile station to transmit a handover complete message (ST2611). Then, the base station B transmits synchronization timing deviation information (synchronization information), scheduling information and signature ID number to the mobile station (ST2612:Message). In addition, in the case that the random access reason is handover, the mobile station is beforehand notified of C-RNTI, and therefore, the base station B does not transmit the C-RNTI.
Upon receiving the information to the mobile station from the base station B, the mobile station corrects the synchronization timing deviation based on the synchronization timing deviation information (synchronization information) (ST2613). Then, the mobile station transmits a handover complete message with radio resources subjected to scheduling to the base station B (ST2614:Message 3). Upon receiving the handover complete message from the mobile station, the base station B transmits a contention resolution to judge whether a collision occurs between mobile stations to the mobile station (ST2615:Message 4).
Thus, since random access is performed also at the time of handover, the collision is inevitable, and it is feared that it will take much time to complete handover. To avoid the fear, proposals not to cause a collision to occur in random access in handover have been made such that the base station assigns a handover random access channel to other physical resources and notifies the mobile station of using the handover random access channel, and that the base station selects a signature for handover to notify the mobile station to perform random access (for example, see Non-patent Documents 4 and 5).    Non-patent Document 1: R1-050850 “Physical Channel and Multiplexing in Evolved UTRA Uplink”, 3GPP TSG RAN WG1 Meeting #42 London, UK, Aug. 29-Sep. 2, 2005    Non-patent Document 2: 3GPP TR (Technical Report) 25.814, V7.0.0(June 2006), Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)    Non-patent Document 3: 3GPP TS (Technical Specification) 36.300, V0.90 (March 2007), Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN), Overall description Stage 2    Non-patent Document 4: R2-063082 “No-contention based handover execution”, 3GPP TSG RAN WG2 Meeting #56 Riga, Latvia, Nov. 6-10, 2006    Non-patent Document 5: R2-063225 “RACH Partitioning for Handover”, 3GPP TSG RAN WG2 Meeting #56 Riga, Latvia, Nov. 6-10, 2006